In the electrophotographic process or electrostatic recording process, two-component developers composed of a toner and carrier particles, and one-component developers composed substantially of a toner alone and making no use of any carrier particles are known as developers for making an electrostatic latent image visible. The one-component developers include magnetic one-component developers containing magnetic powder, and non-magnetic one-component developers containing no magnetic powder. In the non-magnetic one-component developers, a flowability improver such as colloidal silica is often added independently in order to enhance the flowability of the toner. As the toner, there are generally used colored particles obtained by dispersing a colorant such as carbon black and other additives in a binder resin and granulating the dispersion.
Production processes of a toner are roughly divided into a grinding process and a polymerization process. In the grinding process, a thermoplastic resin, a colorant and optional other additives are melted and mixed, the mixture is ground, and the ground product is then classified so as to obtain particles having a desired particle diameter, thereby obtaining a toner. In the polymerization process, a polymerizable monomer composition is prepared by uniformly dissolving or dispersing a colorant, a polymerization initiator and optional various additives such as a crosslinking agent and a charge control agent in a polymerizable monomer, the polymerizable composition is then dispersed in an aqueous dispersion medium containing a dispersion stabilizer by means of a stirrer to form minute droplets of the polymerizable monomer composition, and the dispersion containing the minute droplets are then heated to subject the droplets to suspension polymerization, thereby obtaining a toner (polymerized toner) having a desired particle diameter.
Even when each of these developers is used, an electrostatic latent image is developed with the toner. In general, in an image forming apparatus such as an electrophotographic apparatus or electrostatic recording apparatus, an electrostatic latent image is formed on a photosensitive member evenly charged by exposure to a light pattern, and a toner is applied to the electrostatic latent image to form a toner image (make the latent image visible). The toner image is transferred to a transfer medium such as transfer paper, and the unfixed toner image is then fixed to the transfer medium by a method such as heating, pressing or use of solvent vapor. In the fixing step, the toner is often fusion-bonded to the transfer medium by passing the transfer medium, to which the toner image has been transferred, through between a heating roll (fixing roll) and a press roll to press-bond the toner under heat.
Images formed by an image forming apparatus such as an electrophotographic copying machine are required to improve their definition year by year. As a toner used in the image forming apparatus, a toner obtained by the grinding process has heretofore been mainly used. According to the grinding process, colored particles having a wide particle diameter distribution are liable to be formed. In order for the toner to exhibit satisfactory developing characteristics, therefore, the ground product must be classified to adjust the toner to a limited particle diameter distribution to some extent. However, the classification itself is complicated, and its yield is poor, and so the percent yield of the toner is reduced to a great extent. Whereas, the polymerized toner is easy to control its particle diameter without conducting complicated production steps such as classification and it has come to attract attention in recent years. According to the suspension polymerization process, a polymerized toner having a desired particle diameter and a particle diameter distribution can be obtained without need of grinding and classification. However, the conventional polymerized toners have involved a problem that they can not fully meet requirements in recent years, such as high-speed copying, formation of full-color images and energy saving.
In recent years, copying machines or printers of an electrophotographic system have been required to permit not only reduction of demand power, but also high-speed copying or high-speed printing. A step in which energy is particularly demanded in the electrophotographic system is a fixing step conducted after transferring a toner from a photosensitive member to a transfer medium such as transfer paper. In the fixing step, the toner is fixed to the transfer medium by heating and melting it. Therefore, a heating roll heated to a temperature of at least 150.degree. C. is used, and electric power is used as an energy source therefor. There is a demand for lowering the temperature of the heating roll from the viewpoint of energy saving. In order to lower the temperature of the heating roll, it is necessary to make the toner possible to fix at a temperature lower than that heretofore used. Namely, it is necessary to lower the fixing temperature of the toner itself. The use of a toner capable of fixing at a temperature lower than that heretofore used permits lowering the temperature of the heating roll, and on the other hand shortening the fixing time if the temperature of the heating roll is not very lowered. Therefore, such a toner can meet high-speed copying and high-speed printing.
In order to meet requirements of energy saving, high-speed copying and the like from the image forming apparatus in the design of a toner, it is only necessary to lower a glass transition temperature of a binder resin making up the toner. When a toner is made up of a binder resin having a low glass transition temperature, however, the toner becomes poor in the so-called shelf stability because particles themselves of the toner tend to undergo blocking during storage or shipment, or in a toner box of an image forming apparatus, to aggregate.
In recent years, it has been desired to permit formation of bright images in color copying or color printing by the electrophotographic system. For example, in the full-color copying, the mere melting and softening of toners in a fixing step to fusion-bond the toners to a transfer medium are not enough, but it is also necessary to uniformly melt and mix the toner of different colors to mix their colors. In particular, since color images have come to be often used in OHP (overhead projector) sheets for presentations in various meetings or conferences, toner images fixed to such OHP sheets have been required to have excellent permeability through OHP. In order to meet the excellent permeability through OHP, it is necessary for the toners to uniformly melt on a transparent OHP sheet made of a synthetic resin. Therefore, the melt viscosity of each toner at about the fixing temperature thereof must be designed low compared with the conventional toners. Means for lowering the melt viscosity of the toner include a method in which the molecular weight or glass transition temperature of a binder resin used is lowered compared with the binder resins for the conventional toners. In any of these methods, however, the toner becomes poor in shelf stability because the toner tends to undergo blocking.
As a method for obtaining a polymerized toner having excellent fixing ability, it has heretofore been proposed in, for example, Japanese Patent Application Laid-Open No. 136065/1991 to subject a polymerizable monomer containing a colorant and a charge control agent to suspension polymerization in the presence of a macromonomer. The macromonomer is a relatively long-chain linear molecule having a polymerizable functional group, for example, a group containing an unsaturated bond such as a carbon-carbon double bond, at its molecular chain terminal. According to this method, the macromonomer is incorporated as a monomer unit into the molecular chain of a polymer formed. Therefore, many branches attributable to the long-chain linear molecule of the macromonomer are generated in the molecular chain of the polymer. The polymer apparently becomes a high molecular weight polymer due to entanglement of the branches, i.e., the so-called physical crosslinking, so that the offset resistance of the toner is improved. On the other hand, the physical crosslinking by the macromonomer component is different from chemical crosslinking using a crosslinking monomer such as divinylbenzene and is of a loose crosslinked structure, and so the crosslinked structure is easy to be broken by heating. Accordingly, this polymerized toner is easily melted upon fixing using a heating roll and hence has excellent fixing ability. However, the polymerized toner tends to undergo aggregation among toner particles during storage, and is hence unsatisfactory from the viewpoint of shelf stability.
According to the conventional methods for lowering the fixing temperature of a toner and improving the uniformly melting ability thereof, as described above, an adverse correlation that the fixing ability of the resulting toner is improved, but its shelf stability is lowered arises. As a means for solving this adverse correlation, there has been proposed the so-called capsule type toner in which a toner made up of a binder resin having a low glass transition temperature is covered with a polymer having a high glass transition temperature, thereby improving the blocking resistance of the toner to solve the problem of shelf stability.
As a production process of the capsule type toner, for example, Japanese Patent Application Laid-Open No. 173552/1985 has proposed a process in which a coating layer composed of a colorant, magnetic particles or a conductive agent and a binder resin is formed on the surfaces of core particles having a minute particle size by means of a jet mill. As the core particles, there are used particles formed of a thermoplastic transparent resin such as an acrylate resin or styrenic resin. In this publication, it has been reported that according to this process, a toner of multi-layer structure, which has excellent flowability and improved functional characteristics, can be obtained. When core particles having a low glass transition temperature are used in this method, however, the core particles themselves tend to undergo aggregation. In addition, according to this method, the coating thickness of the binder resin is liable to thicken. Accordingly, this method is difficult to provide a toner improved in both fixing ability and uniformly melting ability while retaining its shelf stability.
Japanese Patent Application Laid-Open No. 259657/1990 has proposed a process for producing a toner for electrophotography, in which crosslinked toner particles prepared by suspension polymerization are added to a solution with an encapsulating polymer, a charge control agent and a parting agent dissolved in an organic solvent, and a poor solvent is then added to the resultant mixture to form a coating film of the encapsulating polymer containing the charge control agent and the parting agent on surfaces of the crosslinked toner particles. According to this process, however, it is difficult to obtain spherical particles because the solubility of the encapsulating polymer is reduced by the addition of the poor solvent to deposit it on the surfaces of the crosslinked toner particles. The capsule wall formed on the surface of the crosslinked toner particle according to this process is uneven in thickness, and moreover is relatively thick. As a result, the effects of improving development properties and fixing ability become insufficient.
Japanese Patent Application Laid-Open No. 45558/1982 has proposed a process for producing a toner for developing electrostatic latent images, in which core particles formed by polymerization are mixed and dispersed in a 1-40 wt. % aqueous latex solution, and a water-soluble inorganic salt is then added to the dispersion to form a coating layer formed of fine particles obtained by emulsion polymerization on surfaces of the core particles. However, this process has involved a drawback that the temperature dependence of charge properties of the resultant toner becomes great due to the influence of the surfactant and inorganic salt remaining on the fine particles, and in particular, the charge properties are deteriorated under high-temperature and high-humidity conditions.
Japanese Patent Application Laid-Open No. 118758/1986 discloses a process for producing a toner, in which a composition containing a vinyl monomer, a polymerization initiator and a colorant is subjected to suspension polymerization to obtain core particles, and another vinyl monomer capable of providing a polymer having hydrophilicity at least equal to that of the resin contained in the core particles and a glass transition temperature higher than that of said resin is polymerized in the presence of the core particles to form shell on each of the core particles. According to this process, the vinyl monomer for forming the shell is adsorbed on the core particles to grow them, so that in many cases, it may be difficult to create a clear core-shell structure because the vinyl monomer absorbed in the interior of the core particles is polymerized. Accordingly, this process is difficult to provide a toner sufficiently improved in shelf stability. In addition, in order to create a clear core-shell structure so as to improve the shelf stability, it is necessary to thicken the thickness of the shell.